Method and system for creating fine lines using ink jet technology

ABSTRACT

In a method and system for producing a geometry ( 24, 35 ) of desired dimension on a substrate, successive droplets ( 43 ) of a material are dropped on to the substrate so as to form a pattern ( 22, 32 ) that is of sufficient dimension to accommodate the geometry within a boundary thereof; and a redundant area of the pattern that surrounds an intermediate portion of the pattern defining the geometry is removed. The redundant material may be removed by curing those portions of the pattern that define the pattern and then flushing the surplus; or by ablation of the surplus material.

FIELD OF THE INVENTION

The invention relates to ink jet technology.

BACKGROUND OF THE INVENTION

In the production of electronic basic structures, such as printedcircuit boards, color filters for liquid crystal displays andsemiconductors, a technology which is commonly used involves patternedmasks similar in principle to those used in lithographic printing forpreparation of printing plates.

A substrate is coated with photosensitive material which is then curedby light. The coating is done by simple dipping or by spin coating whenthe layer is desired to be very thin. Light is then projected on to thephotosensitive layer through a mask which was prepared in advance with adesired pattern. The light which is projected on the photosensitivelayer cause it to harden or “cure” at the exposed areas. Thenon-hardened part, not exposed to light, is washed then away, leaving adesired pattern of hardened layer. Owing the miniaturization of circuitboards, or the required optical precision of color filters there is ademand to create lines as thin as 10 micrometers or less. This isachievable by refined masks and light sources as well as improvedcurable materials.

The lithographic process is expensive for two reasons:

-   -   a large portion of the photosensitive material is washed out        after curing; and    -   each pattern requires its own mask which have to be prepared in        advance.

In the printing industry ink jet technology is used to eliminate theneed of printing plates as ink droplets are placed directly on asubstrate without a mask and under digital control. Thus patterns can becreated, changed or replaced on the fly.

Thus, an attractive alternative to masks used to print thephotosensitive curable material in the electronic industry is to printpatterns of such materials using an ink jet mechanism, thus eliminatingthe need for masks and moreover using only a small quantity ofphotosensitive material in comparison to lithography. The smaller amountof material is because it is used only where needed and avoids the needto coat the total surface of the substrate. As some curable materialscan be rather expensive when mass production is considered, it should beexpected that savings using ink jet printing ought to be substantial.

However, printing fine straight lines by ink jet technologies is acomplicated task. FIG. 1 a shows a line created using ink jet printingby sequentially juxtaposing ink droplets 10 along a line. Owing to thecircular shape that the droplets 10 assume on the substrate, the resulton drying as shown in FIG. 1 b will be a jagged line 11 that is not asfine as required and has a non-uniform edge.

The reasons for this are multiple:

-   -   1. Using even tiny droplets it is very difficult to create lines        of less than 20 μm.    -   2. Even if such lines could be achieved by ink jet, the effects        of surface tension and surface energy of the substrate and local        contamination will create unevenness in drop expansion and        increase the non-uniformity of the line.    -   3. When the directionalities of the droplets' placements are        slightly out of phase (a common phenomenon in ink jet printers,        where directionality is varied by few milliradians at least),        the resulting line appears crooked. This is shown in FIGS. 2 a        and 2 b, where very minor variations in the directionality of        droplets 12, along both X and Y axes contribute to the        unevenness of the line edge of the resulting pattern 13.

Furthermore, attempting to use conventional ink-jet techniques,particularly when very thin lines are required, requires expensive,off-line correction of defects such as pin holes which can render a lineto be non functional. This, of course, is all the more critical when thelines are used to form electrically conductive tracks in a PCB sincesuch pin holes may be manifested as open circuits.

Owing to the simplicity of ink-jet techniques, their profusion andlow-price, it would be a significant benefit if ink-jet technology couldbe used to produce the fine lines required for the fabrication ofelectronic devices without being subject to the drawbacks describedabove.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a methodand apparatus for producing the fine lines required for the fabricationof electronic devices using ink-jet technology that are not subject tothe drawbacks described above.

This object is realized in accordance with one aspect of the inventionby a method for producing a geometry of desired dimension on asubstrate, the method comprising:

dropping successive droplets of a material on to the substrate so as toform a pattern that is of sufficient dimension to accommodate saidgeometry within a boundary thereof; and

removing a redundant area of the pattern that surrounds an intermediateportion of said pattern defining said geometry.

The geometry may simply be a fine line and the invention propose twomajor solutions that enable creation of very fine lines, or any othergeometry, of photosensitive curable material.

A first approach employs a hybrid system containing an ink jet printingsystem and a laser system. The ink jet printing system prints an inkwhich is curable by light of a predetermined wavelength (for exampleinfrared curable ink from IR laser or UV curable ink with UV laser). Theinvention proposes three ways of achieving the solution for ink jetdeposition of photosensitive curable material which is followed bycuring it with light.

According to a second approach, parallel lines of reacting materials areprinted, lines or patterns being created at the areas of reaction.

In addition, the invention also provides means to check and repair linedefects that might be due to missing or misdirected drops.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to understand the invention and to see how it may be carriedout in practice, a preferred embodiment will now be described, by way ofnon-limiting example only, with reference to the accompanying drawings,in which:

FIGS. 1 a and 1 b are pictorial representations of a line formed of aseries of juxtaposed ink droplets using conventional ink jet technology;

FIGS. 2 a and 2 b are pictorial representations of a line formed of aseries of juxtaposed ink droplets when subject to additional distortionsinherent in ink-jet technology;

FIGS. 3 a to 3 c are pictorial representations showing successive stagesin the formation of lines produced by an ink jet assisted processfollowed by laser curing;

FIGS. 4 a to 4 g are pictorial representations showing successive stagesin the formation of lines produced by an ink jet assisted processfollowed by etching;

FIGS. 5 a to 5 g are pictorial representations showing successive stagesin the formation of lines produced by a lithographic assisted processfollowed by etching;

FIGS. 6 and 7 are pictorial representations of alternative systemsaccording to the invention;

FIGS. 8 a to 8 d and 9 a to 9 d are pictorial representations showingsuccessive stages in the formation of lines at areas of reaction betweenparallel lines of reacting materials according to alternativeapproaches; and

FIGS. 10 a to 10 d are pictorial representations showing successivestages in the formation of lines produced by ablation of juxtaposed inkjet droplets.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS Combined Use of Ink JetPrinting and of Curing by Focused Laser Beam

As seen in FIGS. 3 a and 3 b, photosensitive curable material isdeposited as a line by ink jet heads containing at least one nozzle. Thephotosensitive curable material is prepared in advance to have suitableviscosity and surface tension, so it will be easily ejectable and willspread satisfactorily over the substrate.

The ink jet prints a series of juxtaposed ink droplets 15 so as to forma pattern 16 that, with all the distortions as described above withreference to FIGS. 1 and 2, leaves sufficient room for a fine line ofdesired width to be cured within the inked pattern. As shown in FIG. 3b, a laser beam of a desired focus, down to few microns, or in thenanometers range, is moved over the wet ink so as to cure or dry it onlyin that portion 17 of the pattern 16 where the beam was projected. Thesurrounding ink residue 18 that is not cured or dried is flushed out soas to leave a fine line 19 with sharp edge on the printed substrate asshown in FIG. 3 c.

The laser beam is of a specific wavelength that is known to cure theink. Some inks are cured by light at the ultraviolet wavelengths, othersare cured by light at the infrared wavelength. It is also possible touse inks that are hardened by light at visible wavelengths, but theprocess will then be complicated by having to be done in the dark.

Such a method can be used for example to print a black matrix for LCDdisplays where very thin straight lines with width of 10 μm have to beprinted. Most manufacturers currently achieve this by lithography. Themethod according to the invention results in a reduction of the amountof photosensitive curable ink required, printing first ink jet lineswhich are only several tens of microns wide and having these lines thenshaped by laser curing the final straight lines with a required width of10 μm. The uncured photosensitive material is then flushed out of thesubstrate by washing.

According to another approach, very fine lines are created usingelectrically conductive polymers. Currently several companies such asEpson of Japan are trying to construct electronic conductors not withconventional conducting materials but with polymers of high electricalconductivity. Such polymers are ejectable by ink jet mechanisms. Ofthese polymers, those that are curable and hardened by a precise lightsource such as a laser can be printed into fine precise lines accordingto the teaching of the invention. The invention thus paves the way toprint conductors with ink jet technology and replace the conventionallithographic techniques.

The same proposed technology of the invention, ink jetting followed bylaser curing can be applied to production of Thin Film Transistor (TFT)electronics used in manufacturing of flat panel displays and moregenerally in other facets of electronic manufacturing. Currently TFTmultilayer structures are produced with lithographic techniques that arewell-known in the semiconductor manufacturing industry. The lithographicprocesses are used to create patterns that allow the creation of thecomplex multilayer structures by selective etching, coating ordepositions. The patterns created for those purposes are not only linesbut also other geometrical forms. According to the invention, suchgeometrical forms can be generated by first ink jetting a gross outline,and then bringing them into a final desired form by focused beam lasercuring. The operation of the laser beam can be continuous or pulsed, andits movement across the area of the ink jetted curable material can beprogrammed. This coupled with programmable intensity control of thebeam, provides a high degree of flexibility in the creation of desiredlines or shapes.

FIGS. 4 a to 4 g are pictorial representations showing successive stagesin the formation of lines produced by an ink jet assisted processfollowed by etching. By such means, it is possible to allow etching of aselective layer within the multilayer structure of a TFT. The figuresdepict the etching of an “active” layer of such a structure. This activelayer might be a conductive material that has to be shaped so thatconductivity will be confined only to parts of the layer. The exampleshows how in successive stages, an active layer 20 is coated on asubstrate 21, whereafter ink jetted patterns 22 of photosensitivecurable material are formed on the active layer (FIG. 4 c). Laser lightis focused on the photosensitive curable material so as to expose apattern of fine lines thereon. Where the laser light strikes thephotosensitive curable material, the lines are cured to form the desiredpatterns 23 (FIG. 4 d). The uncured material is then flushed (FIG. 4 e)and those areas of the active layer that are not covered by curedmaterial are then etched (FIG. 4 e). After etching is completed, thepattern made of the cured material is removed either chemically or usingany other suitable method. This process will result in an active layerformed into a precise pattern 24 as shown in FIG. 4 g. Those familiarwith the art will understand that using the process of the inventionwill save on photosensitive curable material since there is no need tocoat the complete surface of the active layer with photosensitivecurable material as is conventionally done. The process of the inventionwill also simplify removal and cleaning of the photosensitive uncurablematerial, since there is less of it, and thus be less prone to defects.

FIGS. 5 a to 5 g are pictorial representations showing successive stagesin the same process using lithography. Thus, in successive stages, anactive layer 30 is coated on a substrate 31 and layers of photosensitivecurable material 32 (FIG. 5 c) are applied by an ink jet process. Thereis no need to coat the entire area of the substrate since it issufficient that only those areas where lines are to be formed be coatedand this lends itself to ink jet printing. By such means, the quantityof photosensitive curable material used may be significantly reduced ascompared with conventional lithographic processes where the completearea of the substrate is coated by dipping or spinning. A mask 33 isused to expose a pattern of fine lines on the photosensitive curablematerial to light so as to cure the desired patterns (FIG. 5 d). Theuncured areas of the photosensitive curable material 32 are then flushedso as to reveal the pattern of lines 34 (FIG. 5 e). Those areas of theactive layer that are not covered by cured material are then etched(FIG. 5 f). After etching is completed, the pattern made of the curedmaterial is removed either chemically or using any other suitablemethod. This process will result in an active layer formed into aprecise pattern 35 as shown in FIG. 5 g.

In addition to the formation of accurate lines, both of these methodsare applicable to other geometric forms which are first delineated as agap in non-curable, non adhering material, this gap being then printedover with curable material. When the substrate is flushed or washed,only the material in the area of the primary gap will be left on thesubstrate. Thus, in the context of the description and the appendedclaims the term “geometry” is used to imply any filled shape that may beregular or irregular. It is also to be noted that the term “line” isused to imply any extent of length that is straight, angled or curved orany combination of these properties.

Reliability Issues

One of the major obstacles in ink jet printing is the nozzlereliability. Nozzle reliability can come in several forms:

-   -   Soft failure (nozzle does not work but can be restored in a        maintenance process)    -   Hard failure (nozzle is permanently unusable)    -   Missed directionality    -   Smaller drop volume

Having very thin lines with defects can render them non-functional. Inthe case of color filters mentioned above used in an offset printingprocess, even a slight defect in the black matrix cells can result in amixture of two different colors in an adjacent cell rendering defectivethe picture element they filter.

The first method of the invention yields itself to achieving anincreased process reliability, correcting to a degree the abovementioned problems. This is achieved by the systems 40 and 50 shownschematically in FIGS. 6 and 7, respectively and in which likecomponents are referenced by identical reference numerals. Thus, bothsystems 40 and 50 include a printing nozzle 41 (constituting a firstnozzle), downstream of which there is disposed an optical detector 42,which detects whether the printing nozzle 41 did actually eject adroplet. The detection can be done by looking into the path of theprinting nozzle 41 or of a droplet 43 formed thereby as shown in FIG. 6so as to detect an actual droplet “on the fly”, or by looking at thesubstrate for the actually printed dot as shown in FIG. 7. If theprinted dot were not placed or were not placed in the right position, anauxiliary nozzle 44 (constituting a second nozzle) controlled by acontroller 45 which is responsively coupled to the optical detector 42and operates in accordance with a predetermined control algorithm ejectsa second droplet 46 to replace or complete the first droplet 43. A laser47 downstream of the second nozzle 44 cures or dries the printed imageaccording to the desired pattern. The continuity of the process isachieved by the geometry of the correct relative placement of the firstnozzle 41, the detector 42, the second nozzle 44 and the laser 47 andrelative movement between the substrate and the print system. Thedroplets may be cured directly after their placement on the substrate orcured after the ink pattern is completely formed.

Another approach is to employ one or more redundant nozzles for printingthe pattern so as to increase the probability that the geometry of thedesired pattern will appear intact thus providing on the substrate apattern of sufficient dimension for the laser action. This is a farsimpler solution to implement because the exact shape of the ink jettedline is not crucial as long as this line is continuous. The exactshaping may be achieved by the laser curing.

Creating Fine Lines by Using Two Materials that are Cured by ChemicalReaction

FIGS. 8 a to 8 d are pictorial representations showing successive stagesin the formation of lines at areas of reaction between parallel lines ofreacting materials according to a first approach. In this method a firstline 50 with first material is printed, then a second, parallel, line 51of second material is printed which overlaps the first line so as toform an overlapping area 52. The first and second lines are formed ofmaterials that react on contact via a chemical or physicochemicalreaction that cures the lines where they contact (as in epoxy glues)while having no effect on those areas of the two lines that do notoverlap. The reaction is limited therefore to the overlapping area 52only, and the remains, which are not cured, are flushed after acontrolled time (to prevent over curing).

FIGS. 9 a to 9 d are pictorial representations showing successive stagesin the formation of lines at areas of reaction between parallel lines ofreacting materials according to a second approach. In this method, firstand second lines 60 and 61 formed of a first material that may beflushed in known manner are printed on a substrate. A third, parallel,line 62 of a second material is printed which overlaps the first andsecond lines so as to cover an intervening space 63 between the twolines 60 and 61. The second material is such that in its normal state itcures in air after a known curing time that may be influenced by ambienttemperature. Likewise, it may be formed of a material that can be driedor cured in known manner e.g. by light, heat, etc. By such means, thespace 63 between the two lines 60 and 61 is filled by the curable secondmaterial of the third line 62, which is allowed to cure and adhere tothe substrate. The substrate is then washed so as to flush the first andsecond lines 60 and 61 together with the remaining portions of the thirdline 62 with which they overlap. It does not matter if the portions ofthe third line 63 overlapping the two lines 60 and 61 are also allowedto cure providing it remains possible to flush away the first and secondlines entirely since, in doing so, the cured portions of the third linewill likewise be disposed of.

However, if desired, the second material may be such that on contactwith the first material of the first and second lines 60 and 61 itreacts via a chemical or physicochemical reaction that prevents curingof the second material, while having no effect on that area of the thirdline that does not overlap the first and second lines, i.e. within thespace 63. By such means it may be easier to flush away the redundantmaterial.

Creating Fine Lines with Ink Jet by Overlapping Separate Colored Inks

This method applied to color filters only and may be used in conjunctionwith one of the other methods described above to generate many differentshapes. This method uses the three primary colors (RGB) to create theblack matrix used to separate between the colors. The working process issimilar to that described above with reference to FIG. 8.

In the first stage a first RGB color is printed. In a second stage asecond RGB color is printed to overlap the first one. Every two colorscreate a chemical reaction between the overlapping areas such that theoverlapping areas are converted to black. In this way the black matrixis created at every border between colors. In a final stage the colorsare cured or dried. Such a method may be used in color offset printing,for example in the manufacture of color displays such as LCDs, wherebythe three RGB components of each pixel can be printed using an inkjetwhile ensuring that any overlap is black.

Combined Use of Ink Jet Printing and of Ablation by Focused Laser Beam

The principle described above with reference to FIGS. 3 a to 3 b offirst creating a gross line or shape by ink jetting and then tailoringthe precise form by laser can also be employed where the laser beam isused to ablate parts of the ink.

FIGS. 10 a to 10 d are pictorial representations showing successivestages in such a process. A line of material 70 which can be ablated bylaser beam is printed. A laser beam is used to ablate it on all foursides so as to remove the areas 71, 72, 73 and 74. By such means, whatremains on the substrate is a fine and precise line 75 having fine edgesand a desired width. An excimer laser, for example can be used to createcontrolled and precise ablation.

Other Fields Where the Principles of the Invention can be Applied

Although the invention has been described with particular regard to theformation of fine lines, it is to be understood that the principles ofthe invention are applicable to any geometrical shape. For example,while it known to manufacture PCBs using an artwork that defines thecircuit pads and conductive tracks and which is typically used as a maskthrough which the active layer of the PCB is exposed to light, theinvention allows the artwork to be drawn directly on the PCB thusavoiding the need for a mask. In such an approach the active layer(typically copper) is first covered with photosensitive curable materialso that the exposed portions are cured. The uncured material is thenflushed thus exposing all those areas of the active copper layer thatare redundant, allowing these to be etched without affecting those areasof the active copper layer that are to be preserved. In such an approachmuch of the active layer is disposed of.

However, the principles of the invention allow the copper tracks andpads (i.e. artwork) to be drawn directly on to an insulating substrateusing an electrically conductive ink that is applied using inkjettechnology, allowing surplus ink to be removed either by curing thoseareas that are to be preserved and flushing the remainder; or byablating the redundant areas either prior to or subsequent to curing theremaining material. Such an approach requires far less surplusconductive material to be removed.

1. A method for producing a geometry of desired dimension on asubstrate, the method comprising: dropping successive droplets of amaterial on to the substrate so as to form a pattern that is ofsufficient dimension to accommodate said geometry within a boundarythereof; and removing a redundant area of the pattern that surrounds anintermediate portion of said pattern defining said geometry.
 2. Themethod according claim 1, wherein the redundant area of the pattern isremoved by: curing an area of the pattern that defines said geometry;and removing all material that is not cured.
 3. The method according toclaim 2, wherein the geometry is cured using a laser beam.
 4. The methodaccording to claim 2, wherein the droplets are cured directly aftertheir placement on the substrate.
 5. The method according to claim 2,wherein the droplets are cured after the ink pattern is completelyformed.
 6. The method according claim 1, wherein the redundant area ofthe pattern is removed by: ablating at least one redundant area of thepattern that surrounds said geometry.
 7. The method according claim 6,wherein a laser is used to create controlled and precise ablation. 8.The method according claim 6, further including curing an area of thepattern that defines said geometry.
 9. The method according to claim 1,further including monitoring the droplets and perfecting a missing orincompletely formed droplet.
 10. The method according to claim 9,wherein said perfecting is effected by one or more auxiliary ink jetprinter heads.
 11. The method according to claim 1, further includingduplicating some of said droplets so as to reduce the likelihood of amissing or incompletely formed droplet.
 12. The method according toclaim 11, wherein said duplicating is effected by one or more auxiliaryink jet printer heads.
 13. The method according to claim 1, wherein thematerial is ink.
 14. The method according to claim 1, wherein thematerial is applied only to discrete areas of the substrate that areeach sufficient to accommodate one or more lines within their respectiveboundaries.
 15. The method according to claim 1, wherein the material isa photosensitive curable material.
 16. The method according to claim 15,further including: exposing the photosensitive curable material to lightvia a mask so as to cure the material corresponding to said geometry.17. The method according to claim 15, wherein the photosensitive curablematerial is applied to an active layer on a substrate, there beingfurther included: exposing the photosensitive curable material to lightvia a mask so as to cure the material corresponding to said geometry.flushing the uncured areas of the photosensitive curable material so asto reveal the geometry; etching those areas of the active layer that arenot covered by cured material; and removing the pattern made of thecured material.
 18. The method according to claim 16, wherein thephotosensitive curable material is applied only to discrete areas of theactive layer on said substrate that are each sufficient to accommodateone or more geometries within their respective boundaries.
 19. Themethod according to claim 1, including: inkjet printing a first geometrywith first material, inkjet printing a second, substantially parallel,geometry of second material so as to overlap the first geometry so as toform an overlapping area; the first and second geometries being formedof materials that react on contact so as to cures the geometries wherethey contact while having no effect on those areas of the two geometriesthat do not overlap; and flushing the first and second materials thathave not cured.
 20. The method according to claim 1, including: inkjetprinting substantially parallel non-contiguous first and secondgeometries and formed of a first material that may be flushed from thesubstrate; inkjet printing a third, substantially parallel, geometryformed of a second material so as to overlap the first and second linesso as to cover an intervening space between the first and second lines;curing the second material in the space between the first and secondgeometries so that it adheres to the substrate; and washing thesubstrate so as to flush the first and second geometries together withremaining portions of the third geometry with which they overlap. 21.The method according to claim 1, including: inkjet printing a first areawith first material to form a first primary color, inkjet printing asecond area with a second material to form a second primary color so asto overlap the first primary color so as to form an overlapping area;the first and second materials being such that they react on contact soas to turn black.
 22. A method for producing a fine line of desiredwidth on a substrate, the method comprising: dropping successivedroplets of an ablatable material on to the substrate so as to form apattern that is of sufficient dimension to accommodate within a boundarythereof a line of said desired width; and ablating at least oneredundant area of the pattern that surrounds said line.
 23. An inkjetprinting system comprising: a first nozzle, an optical detector disposeddownstream of the first nozzle for detecting whether the first nozzleplaced a first droplet of inkjet material in an acceptable manner, anauxiliary nozzle disposed downstream of the optical detector, acontroller coupled to the optical detector and to the auxiliary nozzleand being responsive to the optical detector detecting that the firstnozzle did not place a droplet of inkjet material in an acceptablemanner for controlling the second nozzle to eject a second droplet toreplace or complete the first droplet, and a laser disposed downstreamof the second nozzle for curing or drying a pattern formed within aboundary of successive droplets.
 24. The inkjet printing systemaccording to claim 23, wherein the optical detector is configured tolook into a path of the first nozzle or of a droplet formed thereby soas to detect an actual droplet “on the fly”.
 25. The inkjet printingsystem according to claim 24, wherein the optical detector is configuredto look at the substrate for a printed dot.